Fuel Tank pressurized?

[Tyler]

We just had our first 90 degree day here in Georgia and I drove the truck about 110 miles.  Went to get gas and the cap nearly came out of my hand when the tank vented.  I've had the truck for several years, even drove across country and this has never happened.  However, this IS the first hot weather day since installing Holley FI.  I thought the roll-over valve would have kept the tank vented for small pressure changes.

Some insight or knowledge upgrade would be appreciated, thanks!

[Gary Lewis]

If the vent hose that connects to the rollover valve is plugged it can’t vent. I’d check the hose.

[85lebaront2]

First. after 1986 Ford started using larger tank vent systems, probably since everything was going to be EFI by 1988 and EFI returns warm to hot gas to the tanks resulting in more evaporation. The over 8500 GVW trucks do not use a sealed system even with EFI as recently as the last of the OBS trucks in 1997. Maybe first check that nothing is obstructed on your vent system.

[Tyler]

Thank you both.  I'll get under there and give that hose a look.

[kramttocs]

I agree on checking the vent but I just wanted to mention that the 460s (I believe only them) had a gas tank cap that has two levels of tabs. The cap also has a pressure warning on it. With the two levels of tabs and pressure it would vent on the first twist but wouldn't blow the cap off.
I could see the 460s having more vacuum but not sure why they'd have more pressure (Bill?) than any other carbed config.
Reason for mentioning it is (if the vent is clear) maybe the conditions that prompted Ford to put these on 460s is being met with your setup somehow.

[85lebaront2]

The hot fuel handling package did essentially what an EFI system does, hot fuel from the engine compartment is returned to the in-use tank creating a fair amount of vapor pressure. Ever tried to find a locking gas cap for one of these?

[Tyler]

Perhaps my roll over valve is too sensitive and shuts when the fuel heats up.  Maybe I could gut it and buy an aftermarket remote unit to install at the end of the hose.  

[Rusty_S85]

Tyler wrote

We just had our first 90 degree day here in Georgia and I drove the truck about 110 miles.  Went to get gas and the cap nearly came out of my hand when the tank vented.  I've had the truck for several years, even drove across country and this has never happened.  However, this IS the first hot weather day since installing Holley FI.  I thought the roll-over valve would have kept the tank vented for small pressure changes.

Some insight or knowledge upgrade would be appreciated, thanks!

Interested to see what you find out.  I have a '82 that I planned on keeping the OE vapor canister system but if it ends up being a difference be nice to know so I can make the changes then vs waiting till later on.

[Tyler]

As of today I haven't had any more issues. Perhaps the fuel sloshed around perfectly enough to cause the rollover valve to shut and it stayed that way until I removed the fuel cap.
I'll keep my eye on it.

Getting a little over 13 MPG at 70 mph by the way.

Thank you for the replies.

[Gary Lewis]

Good deal!  But 13 MPG @ 70 is nothing to write home about.  Do you have more tuning to do?

[Tyler]

I guess I do now.

The sniper is supposed to tune itself.  I haven't looked into tweaking it.

[Gary Lewis]

Does it control the ignition?  Have you tried more initial advance?

[Tyler]

No ignition control for my setup.  I'm using the stock vacuum advance.

I think I have it set somewhere between 6⁰ and 10⁰ when vacuum line is disconnected.

Hard to see the timing marks on the sawtooth scale.

What would you recommend, timing-wise?

[Gary Lewis]

You might check out this post.  We did a lot of work on David's truck and documented it.

[Tyler]

Looks like I've got some learning to do...

Every MPG counts, especially since I'm starting a cross-country trip in the truck in about 2 months.

Thank you!  

[Tyler]

Bumped timing advance to 14⁰ and leaned out cruising band on the sniper to 15.5 AFR.  The default setting was 14.

No pinging under load at any throttle position.

Most recent tank, got 15.3 MPG.  Neato.

Now I need to learn more about setting a better baseline tune so the sniper won't have to trim as much.

Thanks for the info!

Gary Lewis wrote

You might check out this post.  We did a lot of work on David's truck and documented it.

[Rusty_S85]

Tyler wrote

I guess I do now.

The sniper is supposed to tune itself.  I haven't looked into tweaking it.

Only tuning the sniper does is the base fuel map.  Other aspects such as fuel enrichment with cold and hot start conditions are not self tuning along with fuel enrichment on acceleration as well.

Its why everyone strongly recommends to have the sniper tuned.  I will be having one of the best tune mine once I am done with my build.  His site is Mad Science Motorsports, he does remote tuning as well as first time start up tuning as well.

[Gary Lewis]

Ty - Glad it is working for you.  But you may not have enough vacuum advance dialed in.  You should see what your vac advance is giving at various vacuum levels.